1. Field of the Invention
This invention relates to a thermal transfer printer having a thermal head provided with a plurality of heating elements which are selectively caused to heat ink on an ink ribbon so that the molten ink may be transferred to recording paper to effect desired printing thereon. More particularly, it relates to a mechanism for moving the thermal head.
2. Description of the Prior Art
A known thermal transfer printer is shown in FIGS. 3 and 4. It includes a platen 1 against which recording paper 3 is held by a paper holding roller 2. A cassette 4 containing an ink ribbon 5 has a rectangular opening 6 near the middle portion of its end facing the platen 1. The ink ribbon 5 is exposed through the opening 6. The ink ribbon 5 has one end fastened to a takeup roller 7 and the other end thereof is fastened to a supply roller 8. The takeup roller 7 is secured to a shaft 9 which is driven for rotating the takeup roller 7 when printing is done. A cassette holder 10 is secured to a carriage 11. A positioning pin 12 is provided for positioning the cassette 4 in the cassette holder 10. The carriage 11 is supported on a guide shaft 13 slidably along the platen 1. The carriage 11 is moved by a driving wire 16 fastened at both ends to the carriage 11 and extending about a driving pulley 14 and a driven pulley 15. The driving pulley 14 is connected to an electric motor not shown.
A thermal printing head 18 is disposed in a space defined between the ink ribbon 5 and the opening 6 of the cassette 4. The head 18 has a bottom secured to the carriage 11. The thermal head 18 is normally kept away from the ink ribbon 5 by a spring not shown, as shown in FIG. 4. When printing is required, the head 18 is tilted with the carriage 11 toward the ink ribbon 5 by a solenoid or like device. The ink ribbon 5 has a front surface coated with ink and a rear surface facing the thermal head 18 and not coated with ink. When the head 18 is tilted, it contacts the rear surface of the ink ribbon 5 and thereby brings the front surface of the ink ribbon 5 into contact with the recording paper 3 held on the platen 1. The thermal head 18 is provided with a plurality of heating elements which are selectively caused to heat the ink in accordance with a predetermined printing pattern.
The platen 1 is intermittently rotated as shown by an arrow in FIG. 4 to move the paper 3 after each horizontal line of printing has been made. When no printing is done, the thermal head 18 is kept away from the platen 1.
When one uses any such thermal transfer printer, it is important to avoid any waste of the ink ribbon, as it is expensive. Therefore, it is essential to see that the ink ribbon should not be moved as long as the thermal head is away from the platen or in its up position, but should be moved only when it has been pressed against the platen or is in its down position. See, for example, Japanese Patent Publication No. 21471/1982.
Thus, the thermal head is moved repeatedly between its up and down positions. In the known printer, the carriage is moved for moving the thermal head between its up and down positions, as hereinabove described. Therefore, a large force is required for moving the thermal head and a large solenoid or like device is necessarily required. The known printer is, therefore, undesirably large and expensive.
The use of the carriage for moving the thermal head between its up and down positions presents another problem, too. In order to ensure that the thermal head be pressed against the platen when it is in its down position, it is necessary to position the head so that it may project from the end of the ink ribbon cassette facing the platen. The head so positioned makes it difficult to fit the cassette in position.
Another printer known in the art is shown in FIG. 5. It aims at overcoming the drawbacks or disadvantages of the printer as hereinabove described. It is characterized by employing a thermal head supporting member 20 which is independent of a carriage and which is tilted for moving the thermal head between its up and down positions. A driving wire (or belt) is usually fastened to the carriage for moving it and thereby the thermal head along the platen. This printer has, however, a serious drawback. The displacement of the thermal head supporting member 20 relative to the carriage is unavoidable, insofar as they are independent of each other. Its displacement gives rise to vibration and thereby results in printing in a wrong place or line. Therefore, only a print of poor quality can be produced.